Present work relates to attempts to understnad the pharmacokinetics of potentially useful antiviral drugs in vitro, in experimental animal models and in man. Adenine arabinoside (ara-A) is being studied as a candidate agent. Using a biologic assay in tissue cultures, antiviral concentrations of ara-A in body fluids of patients who received ara-A are being determined. An assay for adenosine deaminase in body fluids and tissues measuring the enzyme inactivating ara-A to its less active metabolite, ara-hypoxanthine, has been developed. Interactions between adenosine deaminase and its potent inhibitor developed in the laboratory of the Parke-Davis Co. are being studied.